Thermoplastic polyamides such as nylon 6,6, have been widely used in the formation of mechanical parts and electrical parts for which excellent mechanical characteristics, high heat resistance and good durability are required. Although polyamides have good heat resistance and high mechanical strength, they are deficient in that the impact resistance, notch sensitivity and moisture resistance are poor. The impact resistance of the polyamides can be improved by blending them with another polymer. However, in general, physical blending of polymers does not provide a complete solution to the poor impact properties of polyamides because the polymers which improve the impact resistance of the thermoplastic polyamides are generally immiscible with the polyamide which results in poor adhesion between the polymers of the blend. As a result, interfaces between blend component domains are areas of weaknesses resulting in mechanical failure.
It is difficult, for example, to obtain a good dispersion of a combination of a polar polymer such as a thermoplastic polyamide with a non-polar polymer such as a polyolefin. It is known that the addition of a graft or block copolymer of similar chemical structure to the blend components can improve the quality of the dispersion. These copolymer additives, generally referred to as compatibilizers, are often added as a third component to the blend. Maleic anhydride grafted polypropylene has been suggested as a compatibilizer for polypropylene/nylon blends by Ide and Hasegawa, J. Appl. Polym. Sci., 18, 963 (1974). The compatibilization of polyethylene/polyamide blends with maleic anhydride grafted polypropylene has been reported by Chen et al in Prym. Engng. Sci., 28, 69 (1988). These and similar blends have been studied, and the results reported by Park et al in Eur. Polym. J., Vol. 26, No. 2, pp. 131-136, 1990.
It has been previously proposed to increase the impact strength of polyamides by the addition of modified block copolymers. For example, Hergenrother in U.S. Pat. No. 4,427,828, discloses blends of thermoplastic polyamide with a modified block copolymer.
Another deficiency of the polyamides is their tendency to absorb water which results in the degradation of its desirable properties. The blending of polyolefins with polyamides has been suggested as a method for decreasing the water absorption for such a blend since a portion of the polyamide which absorbs water would be replaced by a polyolefin which is generally hydrophobic. However, attempts to improve the impact strength of polyamides with polyolefins have generally been unsuccessful because the polyamides were incompatible with the polyolefins.
U.S. Pat. No. 4,795,782 (Lutz et al) describes a polymer blend reported to exhibit improved impact resistance, and this polymer composition comprises a polyamide, a functionalized polyolefin and a functionalized elastomer. The functionalized polyolefins are obtained by reacting a polyolefin with an unsaturated mono- or polycarboxylic acid or derivative thereof. Suitable unsaturated mono- or polycarboxylic acids include maleic acid, maleic anhydride, fumaric acid, etc. The functionalized elastomers described in U.S. Pat. No. 4,795,782 are generally functionalized selectively hydrogenated block copolymers of conjugated dienes and vinyl aromatic compounds. The block copolymers are functionalized by grafting the copolymers with a mono- or polycarboxylic acid compound such as maleic acid, maleic anhydride, fumaric acid, etc. Other monomers which are utilized for introducing the functionality include vinyl monomers such as acrylamide, acrylonitrile, monovinyl aromatic compounds (i.e., styrene), vinyl esters, vinyl ethers, etc.
U.S. Pat. No. 4,657,970 (Shiraki et al) describes polymer compositions comprising at least one thermoplastic polymer including polyamides, thermoplastic polyesters, thermoplastic polyurethanes, vinyl alcohol polymers, etc., and at least one modified block copolymer of a monovinyl-substituted aromatic hydrocarbon polymer block A and at least one olefin compound polymer block B, to which has been grafted at least one molecular unit containing at least one member selected from a carboxylic acid group and groups derived therefrom. The modified copolymers and compositions described by Shiraki are reported to have excellent impact resistance, adhesion, paint adhesion, weatherability, resistance to aging, transparency, etc.
It is known that incompatibility problems may be overcome through the use of a compatibilizing agent. A compatibilizing agent is a material which, on a molecular scale, has particular regions which are compatible with each of the incompatible constituent polymers. Such compatibilizing agents typically surround one polymeric phase providing a chemical and/or physical bridge to the other polymeric phase. Insomuch as portions of the compatibilizing agent are compatible with each of the constituent polymers, the bonding between the two incompatible polymeric phases is effectively enhanced through this intermediate compatibilizing phase. Such a system of incompatible polymers coupled by a compatibilizing agent results in a material which advantageously combines the more desirable properties of the constituent polymers. Lindsey et al., J. Appl. Polymer Sci., Vol. 26, 1-8 (1981) describe a method of reclaiming mixed immiscible polymers by employing a compatibilizing agent. The system studied was a high density polyethylene (HDPE) and polystyrene (PS) and a styrene-ethylene-butene-1-styrene (SEBS) copolymer (a linear triblock copolymer) as the compatibilizing agent. These ternary blends exhibited a considerable improvement in the balance of mechanical properties over a binary blend of high density polyethylene and polystyrene.
U.S. Pat. No. 4,647,509 discloses a multilayer thermoformable packaging material comprising a first layer of (a) a vinylidene chloride polymer, (b) an incompatible polymer, e.g., polyesters and nylons, and (c) a compatibilizing agent, and a second layer of (a) a blend of an olefin polymer, a styrenic polymer, and a compatibilizing polymer, and (b) scrap material produced from the first and second layers. The compatibilizing polymers for the second layer are preferably block copolymers of olefins and styrene such as copolymers of styrene-butadiene, styrene-butadiene-styrene, styrene-isoprene, etc.